Current rotorcraft (helicopter) transmission systems are designed for a single reduction ratio with the engine optimized at a single rotational speed. These transmission systems have no ability to vary the rotor speed other than changing the engine speed. In fact, recent studies have indicated that changing rotor speed using a transmission can provide significant benefits. A 15% rotor speed reduction can result in as much as 5 dB noise-reduction. In contrast, changing the rotor speed by varying the engine speed may have adverse consequences. For example, varying the engine speed impairs engine fuel efficiency and could force the engine to operate closer to the critical stall boundary speed.
Rotorcrafts and/or tilt wing crafts have two modes of operation: a hover mode and a high-speed forward flight mode. In hover mode, a high rotor speed is desirable to improve maneuverability and load factor. In the high-speed forward flight mode, a reduced rotor speed will significantly reduce the rotor noise and keep the velocity of advancing blades subsonic. Thus, in light of the above, a two-speed transmission is needed.
Various transmission concepts have been considered. These transmissions fall generally into two broad categories: those that are continuously variable transmissions (CVT) and those that are two-speed transmissions. Most CVT's were based on traction drives or friction drives where the power was transferred though non-positive engagement frictional contacts. These drives are relatively large and heavy, and their efficiency is poor. Additionally, reliability is a major concern for this type of drive.
Two-speed transmissions that generally using gears to transmit torque and power have a higher power-density and are more efficient than CVT's. However, shifting from one speed to another speed has proven to be a challenging task—particularly under high power. The shift is usually abrupt and large power changes take place abruptly within the transmission. Either of these characteristics could cause a momentary loss of output power or damage to the transmission or drive train.